With respect to separation of a mixture, there are various techniques for removing substances (e.g., salts) dissolved in a solvent (e.g., water). In recent years, use of membrane separation methods is expanding for the processes with less energy and resource consumption. The membranes for use in the membrane separation methods include microfiltration membranes, ultrafiltration membranes, nanofiltration membranes, and reverse osmosis membranes, and these membranes are being used in the case of obtaining potable water, for example, from seawater, brackish water, or water containing a harmful substance, and in the production of industrial ultrapure water, wastewater treatment, recovery of valuables, etc (refer to Patent Documents 1 and 2).
Most of the reverse osmosis membranes and nanofiltration membranes that are commercially available at present are composite semipermeable membranes, and there are two types of composite semipermeable membranes: one which includes a gel layer and an active layer obtained by crosslinking a polymer, the layers being disposed on a porous supporting layer; and one which includes an active layer obtained by condensation-polymerizing monomers on a porous supporting layer. Among the latter ones, a composite semipermeable membrane obtained by coating a porous supporting layer with a separation functional layer including a crosslinked polyamide obtained by the polycondensation reaction of a polyfunctional amine with a polyfunctional acid halide is in extensive use as a separation membrane having high permeability and selective separation properties.
When separation is performed by a reverse osmosis membrane, it is required to apply a pressure, equal to or higher than the difference in osmotic pressure between feed water side and permeate side, toward the feed water, and particularly required to use high pressure as operation pressure in the case where the solute concentration of feed water is high and osmotic pressure is high. Further, when the amount ratio of permeate to feed water (this is called yield) becomes high, the solute concentration of concentrate becomes high, and thus high pressure is required as operation pressure. For example, in the case of desalination of seawater, since the osmotic pressure of seawater having total dissolved solids (TDS) concentration of 3.5% is about 2.5 MPa, when desalination of the seawater is performed in a yield of 40%, the solute concentration of concentrate is about 6 wt %, and operation pressure having equal to or higher than the osmotic pressure (about 4.4 MPa) of the concentrate is required. In order to sufficiently obtain the quality and quantity of permeate, it is actually required to apply pressure, about 2 MPa (this pressure is referred to as effective pressure) higher than osmotic pressure of concentrate, toward the concentrate side. In general desalination of seawater in the background art, operation is performed under the conditions of a pressure of about 6 MPa to 6.5 MPa and a yield of about 40%. On the other hand, there is an example that a reverse osmosis membrane apparatus is operated at a pressure of 7 MPa or higher to bring feed water having high solution concentration into contact with a reverse osmosis membrane (refer to Patent Document 3).
In Patent Document 4, there is proposed a crosslinked aromatic polyamide reverse osmosis membrane which can suppress the decrease of the salt rejection ratio and improve the performance stability under a high solute concentration condition by setting the percentage of pleats having a diameter of 150 nm or less in pleats having a height of 1 nm to 600 nm and a diameter of 1 nm to 500 nm existing in a separation functional membrane to 60%.
In Patent Document 5, there is proposed a supporting layer of a reticulate structure having high physical durability, in which linkage between polymers is increased by electrostatic interaction due to the addition of inorganic salts to a raw membrane formation solution for a microporous supporting layer.